Stenosed carotid artery results in stroke which is one of the leading causes of death, disability and hospital admission. There is, recently, an increasing interest in carotid artery stenting (CAS) for treatment of cervical carotid artery bifurcation atherosclerotic disease.
Unlike atherosclerotic disease in other vascular locations (e.g. the coronary and lower limb arteries), the main concern regarding CAS is atherothrombotic emboli going into intracerebral circulation, rather than hemodynamic insufficiency, which means that carotid stenosis causes stroke not by reducing blood flow rate but by releasing emboli into the distal circulation. However, the current stents used for CAS are modified from stents used to treat obstructive arterial disease of the coronary or peripheral arteries, which do not address the emboli issue.
Conventional carotid artery stenting (CAS) reduce emboli release from the diseased carotid artery by forming a scaffold over the atherosclerotic plaque. However, the effectiveness of the metal struts of the bare metal stent to confine all loose fragments of the atherosclerotic plaque is unreliable. Modification of the stent design or reducing the cell size of a stent maximally may prevent large size emboli from being released from the atherosclerotic plaque but would not be effective for small to moderate size emboli. The size of plaques released during CAS ranged from 3.6 μm to >5000 μm, which are mostly smaller than the size of a cell in a carotid stent.
Embolic protection device was developed to trap any loose fragments going into the distal circulation during the stenting procedure. Its protection is only available during the procedure. It has been observed that about 40% of the CAS stroke events happen between 7 and 30 days after the stenting procedure. Thus, emboli protection during the procedure only is not sufficient to prevent procedure related stroke. Furthermore, there is no significant reduction of the number of micro-emboli detected by brain magnetic resonance imaging (MRI) for CAS patients with or without embolic protection devices. The current bare metal stents and emboli protection devices cannot fully address the fundamental problem of various sizes of emboli released during and after carotid artery stenting for an unstable atherosclerotic plaque.
The use of covered carotid stents, while offering better protection against emboli to the cerebral circulation, cannot be a proper solution as carotid bifurcation is involved in most cases, as the covered carotid stents will unavoidably jeopardize the perfusion of blood into the external carotid artery (ECA). This may affect the hemodynamics over the carotid bifurcation and might also affect cerebral circulation as it is not uncommon for external carotid artery to develop collaterals into the brain in chronic disease condition.
Accordingly, much effort has been put and some different designs of covered and membrane carotid stents have been suggested, in order to achieve emboli prevention and at the same time maintain the ECA branch blood flow. However, none of them is devoid of complications.
Atherosclerotic disease causing emboli to the distal circulation also happened in aorta. Currently, covered stent namely stent-graft can be used to control the emboli going into circulation. However, if the diseased part of the aorta is close to an important branch, the treatment will become more difficult and may involve additional surgical bypass procedure.